Method and apparatus of interconnecting with a system board

ABSTRACT

A method and apparatus of interconnecting with a system board is presented. A system board having a metal stiffener mounted thereon is provided with an opening in the stiffener to provide access to an area of interest on the system board. A probe test assembly is positioned a the opening and secured to the stiffener when testing is desired to provide access to the pins of the device under test (e.g., a Multi Chip Module (MCM) on the system board). Alternatively, a system enhancement device, such as an MCM or Single Chip Module (SCM) having additional Central Processing Units (CPU&#39;s) or other features, may be installed on the system board at the opening in the stiffener to enhance the function of the system board. Another alternate includes an interface assembly positioned at the opening in the stiffener. A cover is positioned at the opening and secured to the stiffener at all other times.

BACKGROUND OF THE INVENTION

[0001] This application is a divisional of U.S. patent application Ser.No. 09/527,577, filed Mar. 16, 2000, which was a divisional of U.S.patent application Ser. No. 09/143,228, filed Aug. 28, 1998, entitled“Method and Apparatus of Interconnecting With a System Board”, theentirety of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a method and apparatus ofinterconnecting with a system board for the purposes of testing and/orimplementation of engineering changes. More specifically, the presentinvention relates to an interconnection scheme where access is providedto an area of interest on the system board for probing and/or connectingto signals on a system board or a component thereon.

[0003] In the testing of large systems during the initial bring up andincluding debugging of system hardware, special modifications aretypically made to the product. A metal stiffener used to support thelarge system boards is machined so that an open access is provided toe.g., pins of a Multi Chip Module (MCM) as well as providing access toother points of interest. There are presently two methods used tomeasure system operations; destructive and nondestructive measurementtechniques. These are accomplished either by direct soldering of probeconnectors to the system board or by the use of an insulated templateand probe arrangement. The first method, direct soldering, provides goodhigh frequency measurements but has many limitations and disadvantages.These limitations and disadvantages include, for example, therequirement that the board must be removed from the test fixture eachtime a connection is to be soldered on, the number of connectionspresent at any time is limited and the connections are susceptible tomechanical failure (e.g., such as being broken off). The second method,utilizing the probe template, offers a full range of interconnections,by means of holes drilled through a template made of an insulatingmaterial, at all signal locations as well as selected ground or voltagereference locations of the MCM. This arrangement is limited tomeasurements in the 500 MHZ bandwidth region. Thus, while this templatearrangement is adequate for error injection and some mid-frequency a.c.measurements, it is not suitable for analysis of high frequencyswitching noise and circuit operation verification.

[0004] Another common problem related to system boards lies inimplementing system upgrades and functional enhancements of the systemboard. Presently such system upgrades and functional enhancementsrequire the system board to be replaced. This leads to expensivecomponent rework, handling, and significant impact of computeravailability at both the development lab and customer's office.

[0005] Still another problem related to system boards is that in theinitial bring up of a machine, it is sometimes necessary to temporarilychange or repair a nets' termination. Present methods include adestructive mechanical solution of soldering terminating resistors, tiedown to ground or a tie up to a voltage on the system board. Again, anytime that a component needs to be attached to the system board, thesystem board must be removed from the test fixture. This impacts testtime, availability of the machine, and the overall schedule of aproduct's development.

SUMMARY OF THE INVENTION

[0006] The above-discussed and other drawbacks and deficiencies of theprior art are overcome or alleviated by the method and apparatus ofinterconnecting with a system board of the present invention. Inaccordance with the present invention, a system board having a metalstiffener (or other such structure) mounted thereon is provided with anopening in the stiffener to provide access to an area of interest on thesystem board. A probe test assembly is positioned at the opening andsecured to the stiffener when testing is desired to provide access tothe pins of the device under test (e.g., a Multi Chip Module (MCM) onthe system board). A cover is positioned at the opening and secured tothe stiffener at all other times.

[0007] The probe test assembly in one embodiment of the presentinvention (high frequency testing applications) comprises an insulatedpattern guide plate and a metal (conductive) probe plate which arepositioned at the opening and secured to the stiffener by an insulatedframe. The insulated frame insulates the metal probe plate from thestiffener. The plates have a pattern or array of holes corresponding tothe pattern of pins on the MCM (i.e., the device under test). Theinsulated pattern plate protects ground pins in the probe plate frombeing exposed. In high frequency applications the metal probe plate ispart of the measurement system. The metal probe plate has resilientground terminals pressed into selected holes therein which provide a lowimpedance ground return path for test measurements. For low bandwidth ord.c. testing applications the pattern plate is eliminated and the probeplate is comprised of an insulation material, whereby the probe platedoes not form part of the aforementioned ground return path. Since theprobe plate in this alternate embodiment is non-conductive a ground pinis not provided.

[0008] Alternative, a system enhancement device, such as a MCM or SingleChip Module (SCM) having additional Central Processing Units (CPUs) orother features, may be installed on the system board to enhance thefunction of the system board, providing the system board has reservedI/O interfaces at the location of the opening in the stiffener. Theenhancement device is retained by a frame which is mounted to thestiffener after the cover has been removed.

[0009] In accordance with another alternate embodiment of the presentinvention an interface assembly is positioned at the opening in thestiffener, after the cover has been removed, and is retained and locatedthereat by the frame. The interface assembly provides for system boardengineering change capabilities and functional upgrade capabilities,providing that the system board has reserved MCM pin locations and sparenets which are prewired in the system board. The interface assemblycomprises an interface board and an interconnect printed circuit board.A pattern or array of holes corresponding to the pattern of I/Ointerfaces (pads) in the system board are provided through the interfaceboard. Resilient coaxial probe connectors (pins) are located in selectedholes for connecting to signal pads. Double ended ground pins arelocated in selected holes for providing a return or ground connection. Aconnector is connected to signal and ground traces/pads on theinterconnect circuit board and is receptive to a mating connector toprovide access to this signal and ground pair for testing (or otherpurposes).

[0010] The above-discussed and other features and advantages of thepresent invention will be appreciated and understood by those skilled inthe art from the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Referring now to the drawings wherein like elements are numberedalike in the several FIGURES.

[0012]FIG. 1 is an exploded perspective view of a stiffener with probetest assembly in accordance with the present invention;

[0013]FIG. 2 is an exploded perspective view of a stiffener with a coverin accordance with the present invention;

[0014]FIG. 3 is a perspective view of the cover of FIG. 2;

[0015]FIG. 4 is an exploded perspective view of the probe test assemblyin accordance with an embodiment of the present invention;

[0016]FIG. 5 is a partial enlarged perspective view of the probe testassembly of FIG. 4 with a system board;

[0017]FIG. 6 is an exploded perspective view of the probe test assemblyin accordance with an alternate embodiment of the present invention;

[0018]FIG. 7 is a perspective view of the probe assemblies of thepresent invention;

[0019]FIG. 8 is a perspective view of a system enhancement assembly inaccordance with the present invention;

[0020]FIG. 9 is a perspective view of an interface assembly inaccordance with the present invention; and

[0021]FIG. 10 is a partial section view of the interface assembly ofFIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0022] Referring to FIGS. 1 and 2, a metal stiffener 10 used to supporta large system board 11 (FIG. 5) has an opening 12 defined (e.g,machined) therein. The opening is also referred to herein as a manhole.The use of a metal stiffener (or other supporting structure) to supporta large system board is well known. The opening 12 in the stiffener 10is located to provide access to an area of interest on the large systemboard 11, such as the pin side of a Multi-Chip Module (MCM), not shown,which is referred to herein as a Device Under Test (DUT). It will beappreciated that the scope of the present invention encompassesproviding access for testing (or other purposes) of any component thatis normally covered by a stiffener and is not limited to an MCM. A probetest assembly 14 (FIG. 1) is positioned at the opening 12 when testing(e.g., a system test, such as when error injection and recovery, isrequired to understand and circumvent system failure mechanisms) isdesired, thereby providing access to the pins of the MCM (i.e., the DUT)as is described hereinafter. A cover 16 (FIG. 2), also referred toherein as a manhole cover, is positioned at the opening 12 at all othertimes to cover the pins of the MCM, thereby serving to protect the pinsof the MCM. The probe test assembly 14 and the cover 16 are preferablyshaped similar to the opening 12, although any shape may be employed. Inthe present example, the probe test assembly 14 and the cover 16 aregenerally square (as is the opening 12).

[0023] Referring now to FIGS. 2 and 3, the cover 16 has opposingsurfaces 18, 20 with surface 18 facing the stiffener 10. The cover 16has four mounting holes 22 therethrough which align with a plurality ofmounting holes 24 in the stiffener 10. The cover 16 is secured onto thestiffener 10 by screws (or other suitable fastening means), not shown,through these mounting holes. a channel 26 is provided about theperiphery of the cover 16 in the surface 18. This channel 26 may bedefined by a plurality of intersecting channels as shown in FIG. 3 or bya continuous channel. Electromagnetic Control (EMC) shielding betweenthe cover 16 and the MCM is provided by a compressible EMC gasket 30mounted in the channel 26. When the cover 16 is mounted by the screws tothe stiffener 10 the gasket 30 is compressed and the effects of EMCnoise scattering is minimized. The cover 16 may also provide mechanicalsupport structure, if such is required as a result of the opening 12weakening the stiffener 10. The cover 16 is preferably comprised of thesame material as the stiffener 10. A plurality of spacers or standoffs31 are provided at surface 18 to structurally reinforce the system board11 which may have been weakened by the removal of material in thestiffener 10 when the opening 12 was provided.

[0024] Referring to FIGS. 4 and 5, the probe test assembly 14 comprisesa frame 32, a pattern plate 34, and a probe plate 36. The frame 32 hasopposing surfaces 38, 40 (FIG. 1) with the surface 38 facing thestiffener 10. A plurality of alignment pins 42 are mounted in holes 44of the frame 32 and extend away from surface 38. The pins 42 arereceived in corresponding alignment holes 46 (FIG. 1) in the stiffener10 to correctly position the probe test assembly 14 relative to the pinsof the MCM. The frame 32 has four mounting holes 48 therethrough whichalign with the plurality of mounting holes 24 in the stiffener 10. Theprobe test assembly 14 is secured onto the stiffener 10 by screws (orother suitable fastening means), not shown, through these mountingholes. The frame 32 has an access opening 54 therein for providingaccess to the pattern and probe plates 34, 36. The frame 32 ispreferably comprised of an insulation material such as FR4, therebyinsulating the plate 36 from the stiffener 10. The probe test assembly14 of this exemplary embodiment is particularly well suited for highfrequency measurement applications, as described more fully hereinafter.Further, it is an important feature of the present invention that theprobe test assembly 14 provides for nondestructive probing of the MCMpins.

[0025] The pattern plate 34 has opposing surfaces 56, 58 with thesurface 56 facing the probe plate 36. A pattern or array of holes 60corresponding to the pattern of pins on the MCM (i.e., the DUT) areprovided through the plate 34 that provide an insulated guide path for aprobe 61. The pattern plat 34 has a plurality of holes 62 therethroughwhich align with a plurality of mounting holes 64 in the probe plate 36.The pattern plate 34 is secured onto the probe plate 36 by screws 66 (orother suitable fastening means) through these mounting holes. Thepattern plate 34 is preferably comprised of an insulation material suchas FR4. Preferably, the surface 58 includes nomenclature (not shown)indicative of the I/O pins of the MCM inscribed thereon.

[0026] The probe plate 36 has opposing surfaces 68, 70 with the surface68 facing the stiffener 10. A plurality of spacers or standoffs 71 areprovided at surface 68 to aid in positioning the probe test assembly 14relative to the pins of the MCM. The standoffs 71 also serve tostructurally reinforce the system board 11 which may have been weakenedby the removal of material in the stiffener 10 when the opening 12 wasprovided. A pattern or array of holes 72 also corresponding to thepattern of pins on the MCM (i.e., the DUT) are provided through theplate 36. The pattern of holes 60 in the pattern plat 34 may comprise afull compliment of I/O locations in the probe plate 36, thus providingaccess to all locations. Alternatively, the pattern of holes 60 in thepattern plate 34 may comprise a limited number of holes suitable fortesting applications that required multiple testing of a limited numberof signal locations. Such limited testing access would, by design, limitthe incidence of probing errors and possibilities of causing a device tocease functioning, especially in an environment where the device wasmission critical and could not be stopped. A plurality of alignment pins74 are mounted in holes 76 of the probe plate 36 and extend away fromsurface 70. The pins 74 are received in corresponding alignment holes 78in the frame 32 to position the pattern and probe plates 34, 36 on theframe 32 and ultimately relative to the pins of the MCM. The probe plate36 has four mounting holes 80 therethrough which align with a pluralityof mounting holes 82 in the frame 32. The probe plate 36 is secured ontothe frame 32 by screws 84 (or other suitable fastening means) throughthese mounting holes. In high frequency applications the plate 36 ismetal and is part of the measurement system. The metal plate 36 hasresilient ground terminals 86 pressed into selected holes 72 whichprovide a low impedance ground return path for test measurements. Theseground terminals (or pins) 86 provide a permanent return path that isuniform and consistent every time the probe test assembly 14 is used. Anexemplary ground path is shown by the broken line 87 in FIG. 5 whereground pin 86 contacts a ground pad 88 on the system board 11. The probe61 is a high frequency probe which is used to access signal points (ie.,pins of the MCM) through the appropriate hole 60, 72, with the signalreturn path being provided by the close proximity of the ground pins 86.The pattern plate 34 provides a non-conductive mechanical cover of theexposed grounding pins 86 in the metal probe plate 36. As describedhereinbefore, plate 36 is insulated from the stiffener 10 by theinsulating material of the frame 32 to enhance the measurement integritythereby insuring that the noise generated by other package componentsare not coupled in the measurements.

[0027] Referring now to FIG. 6, an alternate embodiment of the probetest assembly of the present invention is shown. It will be noted thatelements common to the above described embodiment are numbered the same,whereby reference should be made thereto for a description thereof. Thisalternate embodiment is particularly well suited for low bandwidth ord.c. testing applications. This probe test assembly 14′ comprises theframe 32 (which is the same as the frame 32 described hereinbefore withreference to FIGS. 4 and 5) and a probe plate 36′. The probe plate 36′is the same as the probe plate 36 described hereinbefore with referenceto FIGS. 4 and 5, with the exception that the probe plate 36′ iscomprised of an insulation material such as FR4, instead of metal,whereby the plate 36′ does not in this alternate embodiment form part ofthe aforementioned ground return path (FIG. 5). Since the probe plate36′ is non-conductive a ground pin is not provided pressed into selectedholes 72. The probe 61 shown in this FIGURE is the signal probe only andis used to access signal points through the appropriate hole 72. Aground probe is also required with low frequency probing, as is furtherdescribed hereinafter.

[0028] Referring to FIG. 7, with a high frequency, i.e., measurementcapability in the 3-9 Ghz range, resilient probe 61 (as described in theembodiment of FIGS. 4 and 5) comprises a probe body 88, e.g., aTektronix 10:1 or 1:1 probe body such as P.N 206-0399099 and206-0398-00. A 50 ohm coaxial resilient double ended probe element 90,e.g. P/N 100547-00 from Interconnect Device Inc. is attached by anadapter 92 to the probe body 88. The probe element 90 is a coaxial probeelement whereby the signal is communicated on a center conductor and thereturn ground is provided by an outer conductor, with these conductorsbeing separated by an insulating material. More specifically, one end 93of the probe element 90 is inserted into an opening 94 at a first end 96of the stepped cylindrical shaped adapter 92. One end 98 of the probebody 88 is inserted into an opening (not shown) at another end 100 ofthe adapter 92, such that the end 93 is electrically connected to theend 98 of the probe body 88. The probe element 90 and the probe body 88are maintained in electrical contact and are physically retained withinthe adapter 92 by a pair of screws 102 which are received in threadedmounting holes 104 in the adapter 92. When the screws 102 are tighteneda slot 105 in the adapter 92 closes on the probe element 90 and theprobe body 88, as is clearly shown in the FIGURE. A coaxial cable 106 isconnected to another end of the probe body 88 by a coaxial connector108, as is well known. The other end of this cable 106 is connected todesired testing apparatus for measuring, recording or analyzing thesignal as dictated by the particular test application. As stated before,this probe 61 permits nondestructive measurements in the 3-9 Ghz rangewith very little disturbance to the signal under investigation, dueprimarily to the short return ground paths provided by the ground pins86, the metal probe plate 36 and the outer conductor of the probeelement 90.

[0029] In the low frequency (including d.c.) probe embodiment (asdescribed in the embodiment of FIG. 6), two probes are required, theprobe 61, described above for measurement (i.e., the signal probe) and asecond probe 61′ for ground connection. The second probe 61′ is of thesame type as the measurement probe 61. A wire 110 having resilientconnections 112 at each end thereof electrically interconnects theseprobes to provide the return ground path. Accordingly, the probe 61would be connected to the pin of the MCM to be measured and the probe61′ would be connected to a ground pin of the MCM. A shorting plug 113is connected to the other end of the probe body 88 of probe 61′ to shortthe ground connection provided by the probe 61′ to the probe body 88 ofprobe 61′, thereby completing the ground circuit when wire 119 isconnected.

[0030] Temporary modifications to the system board 11 or module nets arepossible with the probe test assembly 14 of the present invention. Forexample, a 1:1 probe 61 may be used with a temporary short applied to asignal pin, whereby a tie to ground would then be available. Similarly,any combination of terminations, voltages or grounds may be appliedthrough the probe 61 to the system board or module nets. Misconnection,improper terminations, or the need to override a present termination ofa net or nets for system analysis are very desirable. Temporaryconnection of multiple nets are also possible by using two 1:1 probes 61connected together by a short length of coax cable. This provides theability to DOT OR circuits for a period of time, which is extremelyuseful in the early stages of bring up when the system architecture isused for the first time.

[0031] Alternately, a system enhancement device, such as an MCM orSingle Chip Module (SCM) having additional Central Processing Units(CPU's) or other features, may be installed on the system board 11 toenhance the function of the system board, providing the system board hasreserved I/O interfaces at the location of opening 12. Thisfunctionality of this enhancement device can be made to work with acrypto circuit to insure that an upgrade or other operation isauthorized. Referring to FIG. 8, the enhancement device 113 is supportedon a supporting or carrying substrate 114. A plurality of alignment pins115 are mounted in holes 116 of the substrate 114 are received incorresponding alignment holes 78′ in a frame 32′ (the frame 32′ is thesame as frame 32 described thereinbefore) to orientate the enhancementdevice 113 on the frame 32 and ultimately relative to the I/O interfaceson the system board 11. The substrate 114 has four mounting holes 117therethrough which align with a plurality of mounting holes 82′ in theframe 32′. The substrate 114 is secured onto the frame 32′ by screws 84′(or other suitable fastening means) through these mounting holes. Theframe 32′ has four mounting holes 48′ therethrough which align with theplurality of mounting holes 24 in the stiffener 10, whereby thisassembly is secured onto the stiffener 10 by screws (or other suitablefastening means), not shown, through these mounting holes.

[0032] Referring to FIGS. 9 and 10, in accordance with another alternateembodiment of the present invention an interface assembly 118 ispositioned at the opening 12, after the manhole cover 16 has beenremoved, and is retained and located thereat by the frame 32″ (frame 32″is the same as frame 32 described hereinbefore) in the same mannerdescribed herein with respect to the other embodiments. The interfaceassembly 118 provides for system board engineering change capabilitiesand functional upgrade capabilities, providing that the system board 11has reserved MCM pin locations and spare nets which are prewired in thesystem board. An example of such capabilities is where the MCM on thesystem board 11 is replaced in the field with increased functions ormodifications. These new circuit functions would normally be brought toprededicated I/O pins. The interface assembly 118 is configured toconnect the spare board wires that were previously defined in the systemboard 11 to new module I/O and board locations. The interface assembly118 comprises an interface board 119 and an interconnect printed circuitboard 12. The interface board 119 has a first layer 122 of insulatingmaterial such as FR 4 and a third layer of gold plated brass 124. Layers121 and 124 are applied to layer 122 by vapor deposition or any othersuitable method (such as a layer of sheet brass that is gold plated). Apattern or array of holes 126 corresponding to the pattern of I/Ointerfaces (pads) 128 on the system board 11 are provided through theinterface board 119. Resilient coaxial probe connectors (pins) 130 arelocated in selected holes 126 for connecting to signal pads. The probeconnectors 130 are coaxial whereby there is a center conductor and anouter conductor, which are separated by an insulating material. Doubleended, so-called ‘POGO’ ground pins 131 are located (to preferablydefine a small ground loop with respect to the measured signal) inselected holes 126 for providing a return or ground connection.

[0033] The interconnect circuit board 120 comprises a multi-layerprinted circuit board having pads 132 (which connect with pins 130 and131) at one surface 134 thereof which are connected by vias 136 totraces 138 at various layers of the circuit board 120 and to pads 140 atthe other surface 142 of the circuit board 120. The ground path isdesignated 144 and the signal path is designated 146. A connector 148(e.g., a dual in line pin connector) is connected to signal and groundpads 140 at surface 142 of the circuit board 120. A mating connector(not shown) is interconnected with connector 148 to provide access tothis signal and ground pair for testing (or other purposes).

[0034] The interconnect circuit board 120 as a plurality of holes 150therethrough which align with a plurality of mounting holes 152 in theinterconnect circuit board 120. The interconnect circuit board 120 issecured onto the interface board 119 by screws 154 (or other suitablefastening means) through these mounting holes. A plurality of alignmentpins 156 are mounted in holes 58 of the interface board 119 and extendaway from surface 134. The pins 156 are received in correspondingalignment holes 160 in the frame 32″ to position the interconnectcircuit board 120 and the interface board 119 on the frame 32″ andultimately relative to the locations on the system board. Alignment pins161 are provided for attachment of the frame 32″ in the same mannerdescribed in the above embodiments. The interface board 119 has fourmounting holes 162 therethrough which align with a plurality of mountingholes 164 in the frame 32″. The interface board 119 is secured onto theframe 32″ by screws 166 (or other suitable fastening means) throughthese mounting holes.

[0035] While preferred embodiments have been shown and described,various modifications and substitutions may be made thereto withoutdeparting from the spirit and scope of the invention. Accordingly, it isto be understood that the present invention has been described by way ofillustrations and not limitation.

[0036] What is claimed is:

What is claimed is:
 1. A method of testing a system board having an areaof interest, comprising: removing a cover to provide access to the areaof interest on said system board; positioning a probe test assemblycomprising an electrically conductive probe plate associated with apattern plate, said pattern plate comprised of an insulating material,said probe plate having a plurality of probe holes therethrough whichare aligned with a plurality of guide holes in said pattern plate, saidprobe holes are positioned relative to the area of interest; providing aground connection between said probe plate and a ground at the area ofinterest with a ground pin; inserting through a desired said guide holeand associated aid probe hole a probe, said probe having a coaxial probeelement whereby a center conductor of said coaxial probe elementprovides an electrical connection with a signal at the area of interestand an outer conductor of said coaxial probe element electricallyconnects with said probe plate to provide a return ground path;connecting said probe to an external device for testing of said signal;and replacing said over to prohibit access to the area of interest onsaid system board.